The nucleus accumbens is thought to be important in mediating emotional and motivational behaviour, including aggression. Ferrari et al., writing in the European Journal of Neuroscience, have shown that changes in the nucleus accumbens can occur in anticipation of an aggressive encounter, even in the absence of an external stimulus.

The authors used chronically inserted microdialysis probes to measure the levels of extracellular dopamine and serotonin in the nucleus accumbens in rats. Both dopamine and serotonin have been implicated in modulating aggressive behaviour. When a rat is placed into the home cage of another rat, the 'resident' rat will attack the intruder. This bout of aggression is accompanied by an increase in heart rate and also by a rise in dopamine in the nucleus accumbens.

To investigate the effects of anticipated aggression, Ferrari et al. conditioned rats by placing intruder rats in their cages at the same time every day, for ten days. Each day, the resident rats would attack the intruders. On the 11th day, the levels of dopamine and serotonin in the nucleus accumbens were measured but no intruder rat was introduced, so there was no external cue for aggression.

The rats that had been aggressive on previous days showed a marked increase in dopamine in the nucleus accumbens that started about 20 minutes before the time at which the intruder was normally introduced into their cage. The rise in dopamine lasted for more than an hour. There was also a decrease in serotonin levels in the nucleus accumbens, which began at the time when the intruder was expected.

The increase in dopamine is proposed to represent a 'conditioned stress response', in which a rise in dopamine in the nucleus accumbens might help to prepare the rat for an anticipated stressful or aggressive event. By contrast, the authors suggest that the fall in serotonin levels is associated with the termination of this response when the anticipated event fails to occur. The challenge now is to link these acute monoaminergic changes with the proposal that underlying trait differences in monoaminergic transmission are linked to levels of aggression.